Photosynthesis

Overview of Photosynthesis

Photosynthesis is the fundamental process by which solar energy is converted into chemical energy within chloroplasts, sustaining nearly all life on Earth.

• Autotrophs are organisms that produce their own food from inorganic substances, serving as the primary producers in ecosystems.

• Photoautotrophs use sunlight to synthesize organic molecules from CO2.

• Photosynthesis occurs in plants, algae, certain protists, and some prokaryotes.

Photosynthesis Feeds the Biosphere

• Heterotrophs obtain organic material by consuming other organisms, acting as consumers or decomposers.

• Fossil fuels are ancient stores of solar energy, formed from the remains of organisms.

• Most heterotrophs depend on photoautotrophs for food and oxygen.

Chloroplast Structure and Function

Chloroplasts and Leaf Anatomy

Chloroplasts are specialized organelles found mainly in the mesophyll cells of leaves, where most photosynthesis occurs.

• Chloroplasts likely evolved from photosynthetic bacteria.

• CO2 enters and O2 exits leaves through stomata.

• Veins transport water and sugars throughout the plant.

Internal Structure of Chloroplasts

• Chloroplasts have a double membrane enclosing a dense fluid called stroma.

• Thylakoids are membrane sacs arranged in stacks called grana.

• Chlorophyll, the green pigment, is embedded in thylakoid membranes.

Photosynthesis: Chemical Reactions

Summary Equation

Photosynthesis is a complex series of reactions summarized by the following equation:

• The process is the reverse of cellular respiration.

• Chloroplasts split water, releasing O2 and incorporating hydrogen into sugar molecules.

Redox Nature of Photosynthesis

• Photosynthesis is a redox process: H2O is oxidized, CO2 is reduced.

• It is an endergonic process, requiring energy input from light.

Stages of Photosynthesis

Light Reactions and Calvin Cycle

Photosynthesis consists of two main stages: the light reactions and the Calvin cycle.

• Light reactions (in thylakoids): Split water, release O2, reduce NADP+ to NADPH, and generate ATP by photophosphorylation.

• Calvin cycle (in stroma): Uses ATP and NADPH to convert CO2 into sugar.

Light and Pigments

Electromagnetic Spectrum and Visible Light

Light is electromagnetic energy, and only a small portion (visible light) drives photosynthesis.

• Wavelength determines energy: shorter wavelengths have higher energy.

• Visible light ranges from 380 nm to 740 nm.

Absorption and Reflection of Light

• Different pigments absorb different wavelengths; absorbed wavelengths disappear, while others are reflected or transmitted.

• Leaves appear green because chlorophyll absorbs violet-blue and red light, reflecting green.

Pigments and Spectra

• A spectrophotometer measures pigment absorption.

• Absorption spectrum: Graph of pigment absorption vs. wavelength.

• Action spectrum: Profile of photosynthetic effectiveness at different wavelengths.

• Key pigments: chlorophyll a (main), chlorophyll b (accessory), carotenoids (accessory, photoprotective).

Mechanisms of Light Absorption

Excitation of Chlorophyll

When chlorophyll absorbs light, electrons are excited to a higher energy state, which is unstable and releases energy as heat or fluorescence when returning to ground state.

• Fluorescence is the afterglow from excited electrons returning to ground state.

Photosystems

• A photosystem consists of a reaction-center complex surrounded by light-harvesting complexes.

• Light-harvesting complexes transfer energy to special chlorophyll a molecules in the reaction center.

• The primary electron acceptor receives excited electrons, initiating the light reactions.

Electron Flow in Light Reactions

Linear Electron Flow

Linear electron flow is the main pathway, involving both photosystems and producing ATP and NADPH.

1. Photon excites pigment in PS II, energy transferred to P680.

2. Excited electron transferred to primary electron acceptor.

3. Water is split, electrons reduce P680+, O2 released.

4. Electrons move through electron transport chain, creating proton gradient.

5. ATP produced by chemiosmosis.

6. PS I receives electrons, excites P700, transfers electrons to NADP+ to form NADPH.

Energy Changes in Light Reactions

• Light reactions use solar energy to generate ATP and NADPH, which power the Calvin cycle.

• Energy changes can be illustrated by a mechanical analogy.

Cyclic Electron Flow

• Photoexcited electrons cycle back to the cytochrome complex, producing ATP but not NADPH or O2.

• Cyclic electron flow uses only photosystem I.

ATP Synthesis: Chemiosmosis

Comparison of Chemiosmosis in Chloroplasts and Mitochondria

Both organelles generate ATP by chemiosmosis, but differ in electron sources and spatial organization.

• Chloroplasts: electrons from water, protons pumped into thylakoid space, ATP synthesized in stroma.

• Mitochondria: electrons from organic molecules, protons pumped into intermembrane space, ATP synthesized in matrix.

The Calvin Cycle

Phases of the Calvin Cycle

The Calvin cycle is anabolic, using ATP and NADPH to build sugars from CO2.

1. Carbon fixation: CO2 binds to RuBP, catalyzed by rubisco, forming 3-phosphoglycerate.

2. Reduction: 3-phosphoglycerate is converted to G3P using ATP and NADPH.

3. Regeneration: G3P molecules are rearranged to regenerate RuBP, using ATP.

• For one G3P, the cycle consumes 9 ATP and 6 NADPH.

• G3P is a precursor for glucose, sucrose, and other carbohydrates.

Adaptations in Carbon Fixation

C3, C4, and CAM Pathways

Plants have evolved different mechanisms to optimize photosynthesis and conserve water, especially in arid climates.

• C3 plants: Use rubisco for CO2 fixation, susceptible to photorespiration.

• C4 plants: Incorporate CO2 into four-carbon compounds, minimize photorespiration, important in hot, dry climates (e.g., corn, sugarcane).

• CAM plants: Fix CO2 at night, store it as organic acids, release CO2 during the day for the Calvin cycle (e.g., succulents).

• PEP carboxylase in C4 plants has higher affinity for CO2 than rubisco.

• CAM pathway separates carbon fixation and Calvin cycle temporally, while C4 separates them spatially.

Environmental Impact and Agricultural Applications

• Rising CO2 and temperature may affect C3 and C4 plants differently, impacting agriculture.

• Genetic modification of C3 plants to use C4 pathway can increase yield and resource efficiency.

Summary Table: Photosynthetic Pathways

Additional info: The notes include expanded explanations, definitions, and context for each process and structure, as well as relevant examples and applications for exam preparation.